The electronic article surveillance ("EAS") industry has been evolving for several decades in attempts to deter and detect pilferage and shoplifting in libraries, stores and retail establishments. All such EAS systems employ portal control using exit RF transceivers or else magnetometer scanners at store exits to detect the presence, and thereby unauthorized removal, of passive RF transponder or electrically magnetized or magnetizable ("EM") tags, labels or markers attached to merchandise, items or articles under surveillance. These EAS systems generally operate upon principles of sensing RF or magnetic field disturbance in the portal or exit surveillance zone created by the transceiver or magnetometer scanners.
For an example of an EAS system using a passive diode in an RF transponder tag, reference may be made to U.S. Pat. No. 4,063,229, granted to Welsh and Vaughan and assigned to Sensormatic Electronics Corporation; and, for an EM tag EAS system, to U.S. Pat. No. 3,938,125, granted to Benassi and assigned to Minnesota Mining and Manufacturing Company.
Such EAS systems have been efficacious as evidenced by the worldwide proliferation of the ubiquitous white plastic tags attached to garments and merchandise in retail stores with pedestal scanners at exits. However, such EAS systems have inherent performance limitations and deficiencies due to governmental regulatory, economic and fundamental physical constraints on the field strength and detection sensitivity of the transceiver or magnetometer scanners and the innate inefficiencies of energy conversion, modulation or field disturbance of the passive elements such as diodes in the EAS transponder tag. Hence, the EAS systems have been inordinately susceptible to false triggering and alarms, detuning of tags by close proximity to metal or coins and shielding the attenuation of tag response by moisture or the human body. Moreover, such EAS tags are uniresponsive, having only one binary bit ("0" or "1" or "Yes" or "No") of memory, and are thus known in the industry as "dumb" tags; and, from the inception of EAS, the quest has proceeded for a remotely interrogatable "intelligent" tag having multiple binary bits or codes to discriminate among tags under surveillance and automate inventory control.
Most such attempts to encode EAS tags have involved elaborate schemes with multiple discrete ("MD") or expensive integrated circuit microchip ("IC") transponder devices to receive and retransmit the interrogation or surveillance zone field broadcast by the scanner, usually by frequency modulation ("FM") of harmonic or subharmonic responses from the transponder tag. However, the attemps encountered concomitant excessive cost of fabrication and assembly of the MD and IC components, weak signal responses from energy losses in such frequency conversion and modulation with consequent limited interrogation range and tag misreading or nonresponse due to misorientation in the scanner interrogation field, limited code capacity and unwieldy size for operation at the permissible transceiver operating frequencies allocated by governmental regulatory agencies, as well as the residual EAS tag deficiencies and limitations of false triggering from environmental metal and diode-emulating objects, detuning and moisture or body shielding. In many instances, batteries must be added onboard the FM tags to meet minimal performance criteria, thereby further increasing cost and detracting from reliability and feasible applications due to unpredictable operating life and environmental fragility of such batteries in hostile ambient temperature and moisture conditions.
For disclosures of typical encoded EAS transponder tag system designs, reference may be had, for example, to U.S. Pat. Nos. 3,944,928, granted to Augenblick and Keller and assigned to Microlab/FXR (harmonic communication); 4,364,043, granted to Cole, Eshraghian and Roy and assigned to the The University of Adelaide (near-field subharmonic); 4,463,353, granted to Kuzara and assigned to Ralston Purina Company (animal identification by oscillator field disturbance); 4,471,345, granted to Barrett and assigned to Sensormatic Electronics Corporation (coded portal interrogation); and 4,510,490, granted to Anderson, Kearney and Bretts and assigned to Allied Signal Corporation (mechanically resonating magnetic marker or EM tag).
Meanwhile, other approaches to remote electronic object identification were explored, principally for vehicles such as railway cars, using sonic delay lines or bulk acoustic wave ("BAW") piezoelectric resonators or reverberators as passive apparatus for field disturbance response to roadside oscillators. Owing to the unweildy size, excessive expense, short range, slow processing speed and limited code capacity of such BAW devices, they have not secured widespread adoption for automatic identification ("AI") and have not been employed to any significant degree in EAS applications. Examples of designs of such BAW devices may be found in U.S. Pat. Nos. 3,568,104 and 3,273,146, granted, respectively, to Bailey and Hurwitz and assigned to General Electric Company.
As in the case with encoded FM transponder tags, the BAW device designers approached the problems of remote electronic interrogation by focusing upon the frequency domain of the response.
In the meantime, in somewhat unrelated developments over the past several decades, passive microelectronics filter and delay line components, albeit hard-wired for powered circuitry, having been evolving following the rediscovery and adaptation by John H. Rowen of Bell Telephone Laboratories of surface acoustic wave ("SAW") piezoelectric phenomena first discovered over a century ago by Lord Rayleigh. For disclosure of such a selectively tapped SAW delay line in active, as opposed to passive, circuits reference may be had to Rowen's U.S. Pat. No. 3,289,114, assigned to Bell Telephone Laboratories. Such active SAW devices or SAW Chips typically utilize a piezoelectric substrate onto which a pattern of interleaved or interdigitated electrodes or transducers are deposited or adhered for transduction of electromagnetic energy into ultrasonic SAW energy, and vice versa, thereby affording signal delay in the time, as opposed to frequency, domain.
Shortly after Rowen's discovery, encoded active SAW device design publications emerged for shaping and encoding signal responses by predetermined variance of spacing and arrangement of the electrodes or interdigital transducers "IDT'S"), thereby altering response signal phase and amplitude. For examples of such encoded active SAW devices, reference may be had to the disclosures of U.S. Pat. Nos. 3,376,572, granted to Mayo and assigned to RCA, and 3,551,837, granted to Speiser and Whitehouse and assigned to the U.S. Navy. At about the same time, disclosures were published for substituting reflection gratings or grooves, although unencoded, for IDT patterns, as exemplified by U.S. Pat. No. 3,568,102, granted to Tseng and assigned to Litton, and an article by Williamson, Melngailis and Dolat of Massachusetts Institute of Technology, entitled "Reflective-Array Matched Filter For a 16-Pulse Radar Burst" and published in 1975 Ultrasonics Symposium Proceedings, IEEE Cat. No. 75 CHO 994-4SU. While the aforesaid Williamson, et al., reference does describe varying reflection groove depths to impart response signal weighting in an active SAW device, neither it nor any other groove or grating SAW device disclosure of which applicant is aware discloses or suggests binary encoded passive reflective grating or groove SAW Chip transponder devices.
Indeed, early and continued approaches to the present time to incorporate SAW devices into "intelligent" or encoded passive transponder tags for EAS and AI uses steadfastly focus upon varying the spaces between electrode transducers or IDT's. For examples of such designs, reference may be had to U.S. Pat. Nos. 3,706,094, granted to Cole and Vaughan of Australia ("Cole-Vaughan I"), and 3,961,290, granted to Moore and assigned to Texas Instruments Incorporated.
However, apart from the added fabrication costs of laser trimming such IDT's to preselect a code, such SAW Chips with encoded IDT's, at the permissible operating frequencies allocated by regulatory authorities and dictated by size constraints for convenient EAS and AI uses, must have circuit dimensions or line widths under one micron (one millionth of a meter); and such submicron IDT tolerances, being at or beyond the state of the art for mass production of microchip devices, render such SAW Chips with encoded IDT's defect-prone at best. Such limitations have been recognized by parties skilled in the art, whose proposals for alleviating the problem have, however, involved manual field programming of codes onto necessarily inordinately long SAW Chips for sufficient binary bits for EAS or AI use, all as disclosed in U.S. Pat. No. 4,096,477, granted to Epstein and Jordan and assigned to Northwestern University. Nevertheless, as disclosed, for example, in U.S. Pat. No. 4,399,441, granted to Cole and Vaughan and assigned to Unisearch Limited of Australia ("Cole-Vaughan II"), the persistent preference of prior proposers of encoded SAW Chips for passive EAS and AI transponder tags has been for defect-prone IDT's, with defect-tolerant grooves or gratings being relegated to unencodeable, uniresponsive or "dumb" tag resonator transponders for simple or ordinary EAS use. (Comparison is invited of FIG.'S 10 and 11 of Cole-Vaughan I with FIG.'S 9 and 10 of Cole-Vaughan II.)
Hence, production problems persist in the prior art. Moreoever, use of multiple IDT's for encoding diminishes the energy conversion efficiency of a SAW Chip in repeated acoustic to electric effects, thereby decreasing the SAW transponder tag range and detectable binary code capacity.